1. Field of the Invention
The subject matter of this invention is related generally to electromagnetic contactors and more specifically to apparatus for controlling an electromagnetic contactor.
2. Description of the Prior Art
Electromagnetic contactors are well known in the art. A typical example may be found in U.S. Pat. No. 3,339,161 issued Aug. 29, 1967 to J. P. Conner et al. entitled "Electromagnetic Contactor" and assigned to the assignee of the present invention. Electromagnetic contactors are switch devices which are especially useful in motor-starting, lighting, switching and similar applications. A motor-starting contactor with an overload relay system is called a motor controller. A contactor usually has a magnetic circuit which includes a fixed magnet and a movable magnet or armature with an air gap therebetween when the contactor is opened. An electromagnetic coil is controllable upon command to interact with a source of voltage which may be interconnected with the main contacts of the contactor for electromagnetically accelerating the armature towards the fixed magnet, thus reducing the air gap. Disposed on the armature is a set of bridging contacts, the complements of which are fixedly disposed within the contactor case for being engaged thereby as the magnetic circuit is energized and the armature is moved. The load and voltage source therefor are usually interconnected with the fixed contacts and become interconnected with each other as the bridging contacts make with the fixed contacts. In the past the commands which were utilized to control the electromagnetic coil and the contactor came from pushbutton switches which were interconnected electrically with either a 120 volts source of alternating current voltage or DC voltage of a lower magnitude. The contactor had to be rewired to accommodate the different source voltages. It would be advantageous if a contactor could be found which was usable with either an 120 volt AC source or a low voltage DC source. In microprocessor controlled apparatus it is usually necessary to provide some means of dropping the voltage to an acceptable input level which may be in the range of 5 volts. In order to do this with an AC input a solid state clipping and clamping circuit may be utilized for clipping and clamping the high voltage AC input signal. However the clipping and clamping circuit itself usually is protected by a relatively high impedance AC input filter which usually includes a capacitive element. Unfortunately the combination of a capacitive element and high impedance discharge path therefore make it extremely difficult to discharge the filter capacitors after the relatively high voltage command or control signal has been removed. Yet if the capacitor is not discharged it will appear to the microprocessor as representing a voltage source which is indicative of a closed control pushbutton when in fact the pushbutton may be opened. Therefore it would be desirable to find a way to utilize a microprocessor controlled motor controller or contactor which was utilizable on either a relatively low level standard DC control signals or a relatively high level standard AC control signals without having to rewire the circuit for the different rises and without causing the microprocessor to react to false input signals associated with charged filter capacitors.